The present invention relates to a cathode ray tube and a method thereof, and more particularly to a cathode ray tube having a small-diameter neck housing a high performance electron gun and a large-diameter circular array of pins extending through a stem closing one end of the neck and mounting the electron gun thereon, and a method of manufacturing the same.
In general, a cathode ray tube includes a vacuum envelope formed with a panel having a phosphor film coated on its inner surface, a neck housing an electron gun, a funnel joining the panel and the neck, and a stem for closing an open end of the neck and for mounting the electron gun thereon.
In general, six potentials are applied to a color cathode ray tube, including a cathode potential, a control grid potential, an accelerating electrode potential, a focus electrode potential, an anode potential, and a heater potential for heating the cathode.
The heater is formed to pass 200 to 700 mA through two stem pins with a voltage of 5 to 10 V applied between them.
The cathode is supplied with a cathode potential as a display signal to generate an electron beam. The control grid is supplied with a potential of 0 to 200 V.
The accelerating electrode has the accelerating potential of 200 to 1,000 V applied thereto. The focus electrode has the focus potential of 5 to 10 kV applied thereto.
The anode has the anode potential of 20 to 35 kV applied thereto.
The stem pin for applying a high voltage of 5 to 10 kV to the focus electrode is separated from adjacent stem pins a distance of two or more times a regular interval between other two adjacent stem pins to prevent arcing therebetween.
The electron gun structured as described above operates as follows.
The thermoelectrons emitted from the cathode heated by the heater are accelerated toward the control grid by the accelerating potential to form three electron beams.
Each of the three electron beams passes through an aperture of the control grid, an aperture of the accelerating electrode, is focused to some extent by a prefocus lens formed between the accelerating electrode and the focus electrode before entering a main lens formed between the focus electrode and the anode and enters the main lens as accelerated by the focus electrode potential.
The three electron beams are respectively focused by the main lens on a phosphor screen to form a beam spot.
The high voltage to be applied to the anode is supplied via a so-called anode button embedded in the funnel forming an envelope of the cathode ray tube.
The prior cathode ray tube of the type was disclosed in the Japanese Patent Application Laid-Open No. 59-215640.
The prior cathode ray tube having the electron gun described above has the disadvantage that resolution at the periphery of the screen (phosphor film) is lowered as compared with that at the central area.
A chief cause of the lower resolution is astigmatism enhanced due to non-homogeneity of magnetic fields of a self-convergent deflection yoke used generally for scanning the phosphor screen by the electron beams. Another chief cause of the lower resolution is that a focusing condition of the electron beams at the central area of the screen is different from that at the periphery since a distance from the main lens to the periphery is longer than that to the central area.
To solve the problem of lower resolution at the periphery of the screen, it is disclosed in the Japanese Patent Application Laid-Open No. 61-250933 that a focus electrode is divided into at least a first focus electrode and a second focus electrode to form an electrostatic quadrupole lens on their opposing ends and to apply on one of the first and second focus electrodes a voltage dynamically varying according to an angle of deflection of the electron beams.
However, to apply the dynamically varying voltage to a focus electrode, an additional stem pin is required. A problem arises that the increased number of stem pins on a limited size of the stem results in decrease in intervals between the adjacent stem pins so that potential differences between the stem pins is prone to cause arcing therebetween, and a withstand voltage characteristic deteriorates.
A cathode ray tube is proposed in the Japanese Patent Application No. Hei 6-180237 filed in the Japanese Patent Office on Aug. 1, 1994, assigned to the same assignee as the present application, but not laid-open at the time of filing of the present application, wherein to solve a problem of degradation in resolution at the periphery of the screen, a focus electrode is divided into two electrodes to form an electrostatic quadrupole lens therebetween and to apply different voltages on the two focus electrodes, but an electrical interconnection of one end of a heater and a control grid makes an additional stem pin unnecessary despite an increase in the number of electrodes, as described below referring to FIG. 10.
FIG. 10 depicts a cross-sectional view illustrating an electron gun having a focus electrode divided into first and second focus electrodes and voltages applied to the electrodes. In the figure are indicated the heater 21, the cathode 22, the control grid 23, the accelerating electrode 24, the focus electrode 25, the first focus electrode 251 and the second focus electrode 252, and the anode 26.
One end of the heater 21 and the control grid 23 are connected to a common stem pin.
In the figure also are indicated the potential difference Ef across the heater 21, the cathode potential Ek, the control grid potential Ec1, the accelerating electrode potential Ec2, the first focus electrode potential Vf1 and the second focus electrode potential Vf2, and the anode potential Eb.
One of the first focus electrode potential Vf1 and the second focus electrode potential Vf2 is a dynamic voltage that varies in synchronization with a deflection angle of the electron beams.
The other end of the heater 21 that is not connected to the common stem pin connected with the control grid has the potential difference Ef or -Ef applied thereto with respect to the variable control grid potential Ec1. The potential difference applied across the heater 21 therefore is constant even if the variable control grid potential Ec1 changes.
Since the focus stem pins for giving the potential Vf1 to the first focus electrode and for giving the potential-Vf2 to the second focus electrode are at far higher potentials than the other stem pins for giving the required potential to the other electrodes, the focus stem pins are separated from adjacent stem pins a distance of two or more times a regular interval between other two adjacent stem pins to prevent arcing between the focus pins and the other stem pins.
As described above, a focus electrode is divided into two electrodes to form an electrostatic quadrupole lens therebetween and to apply different voltages on the two focus electrodes, but an electrical interconnection of one end of a heater and a control grid makes an additional stem pin unnecessary despite an increase in the number of electrodes, thereby avoiding narrow intervals between adjacent stem pins to prevent deterioration of withstand voltage characteristics which result in arcing between adjacent stem pins.
For example, a neck having an inside diameter of not smaller than 19.1 mm but smaller than 23.1 mm of a cathode ray tube is sealed with a stem having a circular array of stem pins arranged on a circumference of a diameter smaller than 12.2 mm. A circle for stem pins to be arranged on may be called a pin circle hereinafter.
FIG. 7 depicts a partial cross-sectional view illustrating a major portion of a vacuum envelope of a cathode ray tube. In the,figure are indicated the vacuum envelope, the so-called bulb 1, a panel 2, a phosphor film 3, a neck 4, a funnel 5, an electron gun 6, and a deflection yoke 7.
The vacuum glass envelope 1 is formed of a panel 2 on its front side having a phosphor film 3 on its inner surface, a tubular neck 4 on its rear side, and a cone-shaped funnel 5 joining the panel 2 and the neck 4.
The neck 4 is sealed to a small end of the funnel 5. The neck 4 houses the electron gun 6 for emitting electron beams.
The electron gun 6 is mounted on a glass stem (not shown). The stem is sealed to an open end of the neck 4.
The electron beams emitted from the electron gun 6 are deflected in two directions, horizontally and vertically, by the deflection yoke 7 mounted near a transitional area between the funnel 5 and neck 4. The deflected electron beams strike nearly the entire area of the phosphor film 3 formed on the inner surface of the panel 2. An example of the deflected electron beams is indicated by a broken line in FIG. 7.